ID of plant growing EVERYWHERE!

We moved into our new home in September and the garden was a bit of a wasteland. Rubble & rubbish everywhere. I cleaned it up, dug it all over, sowed grass seed and created some borders and a rockery. Very pleased with the end result, BUT there is this random plant that keeps growing literally all over my garden now, in the grass, in the rockery, everywhere. When I dig it up it seems like its sprouting from old choped up roots (I guess left in the ground from when I dug it all over?). Here are some pictures...

Can anyone identify it for me please? It is a pest or should I let it grow? Will it look nice or not? Also need to know if its poisonous or anything as I have a little boy who loves to play in the garden. Thanks in advance!

Definitely rosebay willowherb. Quite attractive spires of pinky purple flowers in late summer, but a real nuisance if you ever let it seed. Worth digging up when you spot it because it's quite large and will squash other plants.

Brilliant! Thank you all so much for the info! As it grows quite tall I am going to dig it out of the lawn but leave it at the bottom and sides of the garden where I am also growing little patches of wild flowers so it should fit in quite nicely! To stop it spreading do I just dead head it before it turns to the fluffy seeds? Also, will it come back next year or would I have to let it spread some seeds?

i would love some of this plant.. i like anything that encourages wildlife in.. and my boys loves butterflies and moths.. so be great.. if it can control as stated and not let it seed then i be happy with it.

Be careful gardeningfantic - you will probably end up with a garden full of the stuff and it won't seem so lovely then, and because the seeds are so fluffy and fine they will float all over the place, hope you've got nice neighbours!!

Isn't Rosebay Willow herb supposed to be getting more scarce in the wild? Meadowsweet is my favourite wildplant. Friends gave me a cutting from their meadow years ago, and now I have clumps in my garden.Love it the smell the fluffy heads. Grows beside the pond, in the shade of a well established camellia bush and interspersed with fennel. Heaven on green and white clouds.